The present invention relates to a method and an apparatus for cooling a high pressure, hot gas laden with ash particles, and more particularly to a heat exchanger design for recovering heat from the high pressure combustible product gas produced in a pressurized coal gasifier and utilizing said heat to produce superheated steam.
A number of coal gasification schemes have been developed in the past few years which produce a combustible product gas which can be upgraded to pipeline quality to supplement our nation's natural gas resources. The chemical reactions occurring in these gasification processes typically occur at temperatures ranging from 1350 to 1650 C. Further, pressures in the range of 1.7 to 10 megapascals are required in order to satisfy system requirements. Other gas cleaning and processing steps are required subsequent to the gasification reaction to produce a product gas suitable for pipeline transmission. Prior to these gas cleaning and processing steps, it is necessary to cool the product gas leaving the gasification chamber from a temperature as high as 1650 C. to a much lower gas handling temperature typically on the order of 200 to 350 C.
A major problem associated with cooling the product gas leaving the gasification chamber is the high concentration of molten ash in the product gas. The reduced gas volume associated with the high gas pressure results in extremely high ash loadings. Typical ash loadings encountered in pressurized gasifier heat exchange sections exceed 2500 kilograms ash per hour per square meter of flow area as compared to typical ash loadings of 50 to 250 kilograms ash per hour per square meter of flow area in conventional coal-fired power plant heat exchanger sections. Therefore, precautions must be taken to avoid plugging of the heat exchanger with accumulated ash deposits which would adversely affect the heat transfer and pressure drop through the heat exchange section.